Reciprocating compressors typically include one or more pistons that “reciprocate” within closed cylinders. They are commonly used for a wide range of applications that include, but are not limited to, the pressurization and transport of air, natural gas, and other gases and mixtures of gases through systems that are used for gas transmission, distribution, injection, storage, processing, refining, oil production, refrigeration, air separation, utility, and other industrial and commercial processes.
Reciprocating compressors are positive displacement machines wherein a reciprocating piston moves back and forth within a fixed cylindrical volume. As such the compressor's capacity is in direct relation to the fixed geometry built into the compressor cylinder(s). The capacity is a function of the compressor cylinder displacement, defined as the area of the piston end face multiplied by the length of the stroke of the piston and the fixed internal clearance volume remaining in the end of the cylinder when the piston is at the outer end of its stroke.
The compressor capacity can be changed by changing the internal clearance volume. There are various common means available for changing this clearance volume. One such device, often referred to as a fixed volume clearance pocket, typically adds a discrete amount of pocket clearance volume to the fixed internal clearance volume that is switched on or off by an actuating device, either manually or automatically.
Another device, commonly referred to as a variable volume clearance pocket, is often located in and on the compressor cylinder outer head. This device incorporates an internal clearancing piston that can be manually moved, using a screw mechanism, to add clearance volume to, or subsequently remove clearance volume from, the head end fixed internal clearance volume. Adding clearance volume reduces the compressor capacity, and removing clearance volume increases the compressor capacity.
Since the required power for the compressor is directly dependent on the capacity of the compressor, such devices are commonly referred to as unloaders, which can reduce the capacity and therefore “unload” the compressor; or they can increase the capacity and therefore “load” the compressor. The variable volume clearance pocket is one of the most effective means of changing the compressor capacity and the required power because it can be positioned at an infinite number of positions or steps within the range of fixed clearance volumes that it is designed to add. Such devices have been in use throughout the compressor industry for many years.
In many compressor applications, conditions change often and sometimes fairly rapidly. It is therefore desirable, for optimal efficiency and utilization of the compressor, to adjust the position of the variable volume pocket's clearancing piston to accommodate changes in operating conditions, for example suction pressure, suction temperature, discharge pressure, gas composition, available driver power, required capacity or flow rate, or other condition changes. Changes in these operating conditions typically affect the capacity and required power of the compressor. For example, in many cases, upsets or sudden increases in suction pressure make it critically important to add fixed volumetric clearance quickly in order to reduce the compressor's capacity and required power and thus prevent damage to the compressor and driver from overload or from operating outside other permissible or safe operating conditions. Changes in operating conditions may also be needed in order to prevent shutdown, for example by a system that is designed to prevent overload of the compressor or driver.
Generally, however, it is not practical and sometimes not physically possible to manually move the variable volume clearancing pocket piston in response to such changes in operating conditions. First, the internal compression pressures can create large forces that make the clearancing piston difficult to move manually, even with a large wrench or hand-actuated wheel for leverage. Second, because of vibration and motion of the compressor cylinder during operation, and the operator's need to be close to the operating equipment in order to move the clearancing piston, it is often a dangerous, or at least threatening, proposition to manually move the clearancing piston while the compressor is running. In fact, many companies do not permit their operators to move the clearancing piston while the compressor is running. Third, before changing the position of the clearancing piston, it is important to know what effects the changes will have on the compressor. Information such as performance curves or other operating guidance is not often available to the operator to enable a safe resetting of the clearancing piston position while the compressor is operating.
Fourth, few compressors are attended by an operator at all times of the day or night, making it impractical to be aware of the need to move the clearancing piston and then to move the clearancing piston in order to unload or load the compressor. Fifth, moving the clearancing piston requires breaking loose a typically large jam nut that locks the screw threads on the manual actuator stem and overrides the cyclic loading imposed on the threads by the cyclic compressor cylinder pressure. The jam nut prevents movement of the clearancing piston, but must be released before the clearancing piston can be moved. Then, after the clearancing piston is moved to the desired setting, the jam nut must again be locked securely to keep the clearancing piston from movement caused by vibration and cyclic pressures acting on the clearancing piston and actuator stem threads.
In virtually all applications, when compressors shut down unintentionally, revenue is lost. In some cases the effects of a compressor shutting down at the wrong time can have catastrophic results when the compressor is part of a complex process. Once shut down, restarting a compressor can take anywhere from minutes to a day or more to restart it, depending on the complexity of the application, remoteness of the location, and other factors. Therefore it is very common practice to set and lock the clearancing piston in a position where the compressor and driver are unlikely to be overloaded during high pressure upsets or process excursions. Although this practice provides a conservative operating margin that usually protects the compressor and driver from damage or overloading during upsets, it subsequently results in underloading and underutilization much of the time.
Therefore, there is an important need for a device and means to automatically move the clearancing piston in a manner that is safe, accurate, reliable and effective, while the compressor is operating. With the development of advanced reciprocating modeling software, which is available from some of the current compressor manufacturers and from the inventors' company, it has been possible and increasingly common practice to automatically control compressors by using control algorithms programmed into digital computers or programmable logic controllers to operate one or more cylinder clearancing devices or cylinder end deactivators to control the capacity produced and the power required by the compressor.
Most of the automatically controllable clearancing devices in common use are discrete step devices that add or remove a fixed amount of clearance volume to the internal clearance volume. Although there are several automatic variable or so called infinite step devices that are used in some limited applications, these devices tend to be more complicated, more expensive and less reliable than the automatically controlled discrete step devices, making their use less acceptable and less prevalent than the industry's needs otherwise require, especially in light of high energy and compressed product values.
More specifically, automatic variable volume clearancing devices that are hydraulically actuated have major disadvantages that limit their use. First the hydraulically actuated devices require a hydraulic actuator and a hydraulic pressure delivery system. Hydraulic oil is usually not desirable around compressors because of the concern about leaks or ruptures of lines and hoses, which could cause environmental contamination and fires. In addition, the potential leakage of hydraulic fluid past seals into the process gas is undesirable and often unacceptable. Finally, hydraulic fluid or other liquids are not perfectly incompressible, especially at higher pressures exceeding about 1000 psig. This typically results in minute oscillation of the unloader piston and actuating system, leading to premature wear of sealing elements and leakage of process gas or hydraulic fluid that results in downtime, frequent maintenance and risk of environmental contamination.
In light of this, an important need remains for a reliable, cost-effective actuating device for automatically controlling the position of the clearancing piston in a variable volume clearance pocket without direct human assistance and without a hydraulic fluid actuator system.